Objections to Reinsch‘s Process.—The chief objection to Reinsch‘s process is the possible impurity of the reagents used—both these reagents, even when supplied as pure, being liable to contain traces of arsenic. As met with in commerce, both hydrochloric acid and metallic copper invariably contain minute quantities of arsenic, the former generally containing the larger quantity of that impurity. Although, by purchasing the purest possible reagents, specially prepared for analysis, it may be possible to ensure their freedom from arsenic, yet in all cases they should be tested before using them. Some of the hydrochloric acid should be diluted with distilled water, and gently heated with the copper foil. If no tarnishing or deposit of any kind occur on the metal after a lapse of several hours, the reagents may be taken as pure and the trial of the suspected substance at once made.
Professor Abel has proposed the following process to ensure the purity of the copper and acid: Boil together equal portions of strong hydrochloric acid and a solution of perchloride of iron. While the mixture is boiling immerse the copper foil, which, if pure, will be merely brightened in colour; if impure, a black deposit on the metal is formed.
Fig. 30.—Photo-micrograph of
sublimate of arsenious acid obtained
by Reinsch‘s process, × 250.
(R. J. M. Buchanan.)
Bloxam‘s Method for the Detection of Arsenic.—The late Professor Bloxam suggested an admirable and delicate process for the detection of small quantities of arsenic. The method is, like that of Marsh, founded on the property possessed by nascent hydrogen of forming a gaseous compound with arsenic; but, instead of the hydrogen being generated by the action of dilute sulphuric acid on zinc, Bloxam generates the gas by an electric current.
The wires from the extremities of a battery terminate in small plates of platinum foil, which are plunged into the liquid to be tested, the apparatus being so arranged that the hydrogen gas evolved from the negative pole is collected. The issuing gas is tested in a similar manner to that obtained in Marsh‘s process.
This method of Bloxam‘s is exceedingly delicate, and possesses one great advantage, that no zinc being used, there is no danger of contamination by the use of impure metal; while, as nothing foreign is introduced during the process of testing, the liquid under examination is left pure for the application of other tests if necessary.
Fig. 31.—Dowzard‘s apparatus for Gutzeit‘s test for
arsenic. A and B indicate glass cells or traps which contain
solutions of lead acetate and copper chloride for the purpose of
fixing H₂S and PH₃ which otherwise would react upon the mercuric
chloride spot on the filter-cap. The cells are fitted into one another,
as shown in the figure.
Gutzeit‘s Test.—This test is more sensitive to the presence of minute quantities of arsenic than that of either Reinsch or Marsh. The apparatus devised by Dowzard should be used (Journ. Chem. Soc., vols. lxxix. and lxxx. 463, p. 715), which consists of an Erlenmeyer‘s flask fitted with superimposed cells, containing solutions which will wash or neutralise those gases which would interfere with the accuracy of the result. The following is Dowzard‘s description of the method of using the apparatus: